1. Field of the Invention
The present invention relates to an information reproducing method that uses replaceable optical disks and an information reproducing apparatus that employs the method.
2. Description of Related Art
The use of such optical disks as CD and DVD is now widespread and development of next generation optical disks that use a blue laser also makes rapid progress to meet the demand of users for increasing the capacity of those disks more and more. And, it has become common now for one optical disk player to be provided with functions of not only reproducing information from CD disks or from both of CD and DVD disks, but also recording information on all of CD-R/RW, DVD-RAM, and DVD-R/RW disks. And accordingly, the users have come now to demand the compatibility of recording/reproducing functions among those optical disks manufactured in various standards to be more improved.
The PRML (Partial Response Maximum Likelihood) method, which is remarkably effective for improving the S/N ratio, is now widespread as means for expanding the capacity of magnetic disks. The PRML method compares reproduced signals with a target signal sequentially for a continuous N time to transform each reproduced signal bit array to the most likelihood binary bit array. Conventionally, a direct slicing method has long been employed to reproduce information from optical disks. However, the conventional method has its limit in speeding up the processing and expanding the disk capacity. This is why the PRML method has come to be employed as means for reproducing information from those optical disks.
In spite of such an advantage, the PRML method has been confronted with the following three problems that must be solved before the method is to be used positively as means for reproducing information from optical disks.
The first problem is asymmetry occurrence. A PRML target signal is calculated by a convolution processing of a specified impulse response (PR class) and a bit array, so that the PRML target value becomes symmetrical vertically to the center value. On the other hand, each optical disk usually generates asymmetry to improve the S/N ratio. As a result, the reproduced signal level becomes asymmetrical and hardly matches with the PRML target signal.
The second problem is an impossibility to realize significant expansion of disk capacity. There has never been proposed any method for such significant expansion of disk capacity with use of the PRML method so far. This point will be described later in detail.
The third problem is the minimum run length. The PRML basic structure is decided uniquely by a PR class and the run length of a reproduced signal. However, the minimum run length of MO disks (optical disks in the next generation) is 2T while that of the CD/DVD disks is 3T. Consequently, to enable one optical disk drive to correspond to such a plurality of optical disk types, the drive must also be provided with a plurality of PRML circuits.
Technical Digest of ISOM 2002 (269-271 (2002)) discloses an application example of the PRML method to an optical disk. In the document, a target signal level is changed according to the circumstances to reproduce information from the optical disk while the disk tilts in both radial and tangential directions. This method is excellent, since it can solve the first problem described above (asymmetry occurrence). However, the method is still difficult to solve the second problem (impossibility to realize significant expansion of disk capacity) and cannot solve the third problem (impossibility to process signals having different minimum run lengths in the same way).
[Non-Patent Document 1]
Technical Digest of ISOM 2002 (269-271 (2002))
As described above, the conventional PRML method is hardly effective to expand the capacity of optical disks significantly and cannot process signals having different minimum run lengths in the same way.
Under such circumstances, it is an object of the present invention to provide an information reproducing method that can solve the above conventional technical problems, realize significant expansion of disk capacity, and process signals having different minimum run lengths in the same way, as well as an information reproducing apparatus that employs the method.